1. Field of the Invention
The invention relates to a method and apparatus for the prevention of high temperature corrosion due to alkali sulfates and chlorides in boilers and other fired apparatus.
2. Description of the Prior Art
The problem of high temperature corrosion in boilers and other types of fired apparatus has been appreciated for a very long time. Mainly because the process was not understood, however, attempts to find a solution were empirical or engineering make-do's. There have been several feasible attempts to consider fuel additives (E. Raask, Mineral Impurities in Coal Combustion: Behavior, Problems and Remedial Measures (1985)) and such species as Magnesium, Calcium or Silica based compounds were added to little avail. Instead, for the last 20 years or so, attention has turned to accepting the presence of high temperature corrosion, but minimizing its effect on materials by developing superalloys or using coatings.
Even more recently in the United States, the higher temperature gas turbine developers have given in altogether and now will only burn very clean fuels such as natural gas. Coatings and alloys have been a great help but are only a partial solution as they all degrade with time. Chan, in a paper entitled "Coating Life Prediction under Cyclic Oxidation Combustion" ASME J. Eng. Gas Turbines Power 120:609 (1998) formulates a coating lifetime model. What is more, all coatings and alloys must be prefabricated. They do not regenerate in use, but gradually degrade in an irreversible manner before finally failing. They generally are somewhat exotic and tough materials. The coating described in Edwards, III et.al., "Inhibiting Coke Formation by Coating Gas Turbine Elements with Tungsten Disulfide," U.S. Pat. No. 5,240,741 (1993) is made of tungsten disulfide, and is meant to inhibit deposition of coke in a certain process. These coatings are not flame components and it is not possible to create them in a normal combustion process. They are made by other techniques, prefabricated and then used.
Coatings do not relate to flame chemistry or flame additives in any way. Consequently, patents relating to coatings or alloys are irrelevant to the invention. Just because tungsten metal may be a better metal surface than stainless steel has nothing to do with combustion chemistry. It cannot be made by deposition from any concoction of flame additives.
Morimoto et.al., "Method of Controlling Deactivation of Denitrating Catalyst," U.S. Pat. No. 4,831,942 (1989) did add, along with the major iron component, traces of vanadium and tungsten compounds to pulverized coal and oil fuels. This was intended to improve the performance of a nitric oxide emissions control catalyst that suffered from poisoning problems in the flue gases. Morimoto burned pulverized coal in a boiler, but the conditions do not impact the invention. Indications are, primarily due to the mixture of additive ingredients, that no corrosion protection was evident in Morimoto's boiler. In fact, if anything, indications are that corrosion was worse, and Morimoto had to keep additive levels low to minimize it. Morimoto's process is based largely on an addition of iron compounds that produce iron oxide particles in the burnt gases. Small traces of vanadium or tungsten compounds were found to be beneficial in enhancing the effectiveness of the iron oxide particles. Various statements are made in Morimoto's description, which teach away from the invention. Morimoto's additives enhanced corrosion and seems sensitive to particle size of the iron oxide; column 3, line 66. Vanadium or tungsten are added to the major iron ingredient only in small amounts, column 4, line 30. Adverse effects on the boiler were found due to these additives, column 4, line 37: The additives, sodium vanadate or sodium tungstate are suggested as the compounds of use, column 6, lines 25-31. Use of these compounds will increase the concentrations of sodium in the burned gases and enhance corrosion.
In the invention, gas phase levels of tungsten introduced for corrosion protection into a flame are in excess of those of the alkalis. In Morimoto low levels of tungsten were added along with the vanadium and iron, so that sodium concentrations were in excess of those of tungsten and no corrosion improvements could have been expected. See, column 12, line 64, where I was stated that additions of vanadium and tungsten compounds were kept very small to minimize the corrosion effects.
The invention relates to flame deposition of alkali metal salts, primarily sodium and potassium salts, onto cooled metal surfaces immersed in the burnt gases. This has been one of the first quantitative studies and previously only qualitative engineering reports have been available. Nothing was previously known of the formation mechanisms or what were the controlling parameters. Some preliminary results are summarized in an earlier paper (26th International Symposium on Combustion, 1996) and in a recent Poster Presentation (27th International Symposium on Combustion, 1998). However, these papers do not discuss the claimed invention, but only discuss the background of the invention.